The Lord might have said vengeance is his, but that doesn't stop two Irish brothers newly arrived in Canada from trying to steal it away from Him. Michael O'Brien's wonderfully dark comedy Restitution: An Irish-Canadian Rhapsody follows these siblings in the 1880s and 90s, when the drunken Seamus Macready does in his brother Stephen's son with an ax. Their lives then become a blood feud that's part cat-and-mouse game, part one-upmanship.

"I'm not a theoretician of theatre," smiles the playwright, "but I know that I like to see a theatre of surprises, a good yarn that constantly astonishes me. Issues can be explored accidentally, on the side, but I want to find a shock in each scene, a surprise that's sometimes hilarious, sometimes heartbreaking, sometimes terrifying."

O'Brien's best known for his stage and radio adaptations of fiction, notably works by Charles Dickens and Margaret Atwood. But one of his best works - it has yet to play Toronto - is Mad Boy Chronicle, which takes a savagely funny look at Hamlet. With Restitution, he returns to creating an original work, one he's developed in the Factory playwrights' unit.

He admits there's a bit of mischief in subtitling the piece a rhapsody. One of the characters uses the word to describe his own writing, and O'Brien wants to mock that pretentiousness while endowing what he terms "a rather shocking and outrageous play" with a romantic aura.

And with all the Dickens adaptations under his belt, the playwright also thumbs his nose at Dickens's golden, optimistic Christmas tales.

Talking in a lounge at Factory, we're regularly interrupted by screams and banging on the ceiling, as if a body has gone down heavily above us. Rehearsals are obviously vigorous, and the brothers are apparently having one of many goes at each other as part of the play's physical and verbal slagging.

"There's nothing uniquely Irish about fighting - or drinking, for that matter," says the thoughtful O'Brien, referring to a pair of key elements in the script. "But the Irish do it with delight and joy, and they also have a singular way of suffering great adversity and transforming it into laughter.

"They almost rejoice in tragedy. In Restitution I'm playing with that Irish archetype and also the idea of how personality is shaped by ancestors, even those one's never met."

O'Brien is sometimes asked whether the story he tells is a true one. His response?

"There's no such thing as a true story. As soon as you open your mouth to tell it, you refract whatever reality exists through your own mind.

"Still, every story contains bits of truths floating around in it, either intentionally or not. So I hope there's some truth in Restitution, but no way is it a true story."